The alterations to the organic acid profile of dental plaque from interproximal sites in the dentition of two individuals were monitored over a period of 2 hours following a 5% sucrose rinse. Analyses were performed by capillary gas chromatography and a bacterial luciferase linked lactate dehydrogenase reaction. Acetic, propionic, butyric, isovaleric and lactic acids were routinely analyzed in plaque samples of 1 mg wet weight and below. Following sucrose application the concentration of lactic acid rose to a peak value after approximately 2 minutes. Increases in the concentration of propionic acid after 5-10 minutes and acetic acid after 20-30 minutes were also recorded. By 60 minutes the pattern of composition had returned to pre-sucrose values. Plaques from the less exposed sites in the dentition produced higher concentrations of propionic acid than the sites open to salivary access, possibly as a result of a greater retention and subsequent fermentation of lactic acid at these sites.